1. Field of the Invention
The present invention relates to a wireless communication system. More particularly, the present invention relates to a terminal, method for operating the terminal, and method for interworking in a wireless communication system including a 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) network and 3GPP legacy network.
2. Description of the Related Art
Research is being conducted to develop a next generation communication system. A representative example of the next generation communication system is a communication system based on the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) standard (hereafter referred to as an LTE network). When the LTE network is deployed, it is expected that the LTE network will coexist with a legacy communication system in an overlay mode. A representative example of the legacy communication system is a communication system based on a 3GPP Wideband Code Division Multiple Access (WCDMA) standard (hereafter referred to as a WCDMA network).
When an operator newly deploys the LTE network in the overlay mode, the operator may support a voice service through the WCDMA network and may support a data service through the LTE network. When the voice service is supported through the WCDMA network, the voice service may be referred to as a Circuit Switched (CS) voice service. When the LTE network is deployed in the overlay mode, a terminal for use therein should be a dual mode terminal that supports a Radio Access Technology (RAT) of both the WCDMA network and the LTE network. In addition, the terminal should be capable of receiving paging messages of the WCDMA network and the LTE network.
To support the CS voice service through the WCDMA network and to support the data service through the LTE network, a conventional function referred to as CS fallback may be employed. An example of the conventional CS fallback will be described below with reference to FIG. 1.
FIG. 1 illustrates a system configuration for conventional CS fallback between the LTE network and a WCDMA network.
Referring to FIG. 1, a system configuration for CS fallback between the LTE network and the WCDMA network includes a terminal 100, a Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (UTRAN) 110, a Global System for Mobile communications (GSM) Enhanced Data Rates for GSM Evolution (EDGE) Radio Access Network (GERAN) 112, a Serving General Packet Radio Service (GPRS) Support Node (SGSN) 114, a Mobile Switching Center (MSC) 116, an Evolved-UTRAN (E-UTRAN) 120, a Mobility Management Entity (MME) 122. The terminal 100 is common to both the LTE network and the WCDMA network. The UTRAN 110, GERAN 112, SGSN 114, and MSC 116 are included in the WCDMA network. The E-UTRAN 120 and MME 122 are included in the LTE network.
In the WCDMA network, the terminal 100 communicates with the UTRAN 110 via a Uu interface and the GERAN 112 via a Um interface. The UTRAN 110 communicates with the SGSN 114 via an Iu-ps interface and the MSC 116 via an Iu-cs interface. The GERAN 112 communicates with the SGSN 114 via a Gb interface and the MSC 116 via an A interface. The SGSN 114 communicates with the MSC 116 via a GS interface.
In the LTE network, the terminal 100 communicates with the E-UTRAN 110 via an LTE-Uu interface and the E-UTRAN 110 communicates with the MME 122 via an S1-MME interface.
The LTE network and the WCDMA network are connected to each other through an SGs interface between the MME and the MSC and an S3 interface between the MME and the SGSN.
When the MSC 116 receives a call request voice service, the MSC sends a CS paging message to the terminal 100 through a WCDMA cell in which the terminal 100 is located. However, when the terminal 100 is located in an LTE cell, the MSC sends a CS paging message to the terminal 100 through the LTE cell via the MME 122.
The terminal 100 registers with the MSC 116 to inform the MSC 116 of its location so that the MSC 116 is aware of the location of the terminal 100. When the terminal 100 is within a service coverage area of the LTE network, the LTE network performs tunneling for the CS paging message between the terminal 100 and the MSC 116 of the WCDMA network and for control messages used for a registration area update procedure. When the terminal 100 receives the CS paging message through the LTE network, the terminal 100 releases its connection with the LTE network and connects to the WCDMA network. The terminal 100 sends a CS paging response message through the WCDMA network, after which, the WCDMA CS voice service is initiated. Therefore, as described above, the CS fallback allows the terminal to receive a paging message from the WCDMA network, even when the terminal is connected to the LTE network.
However, when the operator newly deploys the LTE network in the overlay mode, it is expected that the coverage area and signal quality of the early LTE network will be limited compared to the WCDMA network. That is, there will be areas that are serviced by the WCDMA network that are not serviced by the LTE network. In addition, there will be areas that are serviced by the LTE network in which the terminal poorly receives signals from the LTE network. In this environment, there may be a number of problems that are encountered, such as those listed below, when receiving the CS paging message through LTE network using the conventional CS fallback.
One problem with the conventional CS fallback involves deterioration of a paging receiving ratio. This problem is experienced when a paging message is lost due to a low quality signal received from the LTE network. As a result, there may be a delay before a CS voice service of the WCDMA network is established or there may be a failure to establish the CS voice service of the WCDMA network. The CS voice service is one of the basic services of a wireless network, and thus a receiving ratio for the CS voice service is an important element of the quality of service sensed by the user.
Another problem with the conventional CS fallback involves the switching by the terminal from the LTE network to the WCDMA network for a voice call. The CS voice service is provided by the WCDMA network after receiving a CS paging message through the LTE network. To achieve this, the terminal has to switch from the LTE network to the WCDMA network. The switching from the LTE network to the WCDMA network introduces a delay before the start of the voice service. The delay affects the quality of service sensed by the user.
Yet another problem with the conventional CS fallback is that, due to the inconsistent service coverage area of the LTE network, the terminal frequently switches between the LTE network and the WCDMA network. In the conventional CS fallback, it is required that the terminal camp on the LTE network when it is available and only camp on the WCDMA network when the LTE network is not available. In addition, when the terminal is camped on the WCDMA network, the terminal should continuously search for the LTE network and camp on the LTE network once it is found. However, these operations result in a consumption of the terminal's battery power and a reduction of the efficiency of both the WCDMA network and the LTE network.
One technique that may be employed to address the shortcomings of the conventional CS fallback is to employ a hybrid operation on the terminal. The hybrid operation denotes that the terminal takes turns monitoring a paging channel in the WCDMA network and the LTE network. In the hybrid operation, the terminal may maintain a substantially identical receiving ratio because the terminal receives the CS paging messages through an air interface with the WCDMA network. However, in the hybrid operation, the terminal switches to the WCDMA network even when the terminal is sending/receiving data through an active connection with the LTE network. Accordingly, performance of the LTE network is compromised.
Therefore, a need exists for an apparatus and method for enhancing a receiving ratio of the CS voice service with greater reliability than the conventional CS fallback, while not decreasing performance of the LTE network.